1. Field of the Invention
The present invention relates to a brake control apparatus for vehicles.
2. Description of the Related Art
An automatic braking apparatus for automatically controlling the fluid pressure of wheel cylinders independently of the operation of brake operation members, such as a brake pedal, by a driver has been widely known. For example, the automatic braking apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-9914 includes a master cylinder for generating a master fluid pressure (master cylinder fluid pressure and vacuum booster fluid pressure) independently of the operation of a brake pedal on the basis of the operation of a vacuum booster corresponding to the operation of the brake pedal, a fluid pressure pump for generating a pressurizing fluid pressure higher than the master fluid pressure, and a normally open linear electromagnetic valve for adjusting the applied pressure (pressure difference) to the master fluid pressure using the pressurizing fluid pressure by the fluid pressure pump.
In this apparatus, a distance between oneself vehicle having the apparatus mounted thereon and a vehicle ahead is detected, and when the distance is less than a predetermined reference value, the fluid pressure pump and the normally-open linear electromagnetic valve are controlled. The apparatus is to automatically apply a braking force to the oneself vehicle independently of the brake pedal operation by a driver by automatically controlling the braking force based on the fluid pressure (fluid pressure braking force) using “the master fluid pressure having the applied pressure added thereto” generated by the apparatus.
Recently, techniques have been developed in which the above-mentioned automatic brake apparatus is applied to a motorized vehicle using a motor as a drive source and a so-called hybrid vehicle using a motor and an internal combustion engine together as drive sources so that the regenerative and cooperative brake controls using the fluid pressure braking force and the regenerative braking force together are executed.
More specifically, the apparatus establishes boosting characteristics of the vacuum booster so that the master fluid pressure relative to the operation force of the brake pedal (depressing force of the brake pedal) intentionally has a value lower than a predetermined target value. Thereby, “the fluid pressure braking force based on the master fluid pressure (master fluid pressure braking force)” relative to the depressing force of the brake pedal is established so as to intentionally have a value lower than a predetermined target value.
The apparatus is to adjust a compensatory braking force (specifically, the regenerative braking force and the applied fluid pressure braking force) in accordance with the brake-pedal depressing force so that characteristics of the sum of the master fluid pressure braking force and the compensatory braking force including “the regenerative braking force due to the motor” and/or “the fluid pressure braking force based on the applied pressure due to the normally-open linear electromagnetic valve (the increase in fluid pressure braking force to the applied pressure or the applied fluid pressure braking force)” (the sum of the braking forces applied to front wheels and the braking force applied to rear wheels) relative to the brake-pedal depressing force agree with predetermined target characteristics. In addition, the regenerative braking force due to the motor is used with priority over the applied fluid pressure braking force.
Thereby, the characteristics of the total braking force relative to the brake-pedal depressing force agree with the target characteristics, so that the uncomfortable brake feeling of a driver is not generated. Additionally, when the vehicle speed is reduced due to the brake-pedal operation by the driver, the electric energy generated by the motor corresponding to the regenerative braking force due to the motor can be actively recovered to a battery, resulting in good gas mileage of the vehicle by improving the energy efficiency of the entire apparatus.
On the other hand, during the brake-pedal operation by the driver, if the rear wheels are locked ahead of the front wheels, the vehicle running is liable to be unstable. Hence, if the rear wheels are liable to be locked ahead of the front wheels, there is a control for holding the fluid pressure of the rear wheel cylinders (referred to as the front/rear brake power proportioning control below). By performing the front/rear brake power proportioning control, the increase in brake power applied to the rear wheels is prohibited, preventing the rear wheels from being locked ahead of the front wheels.
Techniques have been developed in which to a motorized vehicle having a motor for driving front wheels as a drive source or a hybrid vehicle having a motor, the above-mentioned automatic brake apparatus is applied and the front/rear brake power proportioning control is executed in addition to the regenerative and cooperative brake controls mentioned above.
That is, in this technique, during the brake-pedal operation, the regenerative and cooperative brake controls are executed. Thereby, the brake power applied to the front wheels is controlled by the fluid pressure brake power and the regenerative brake power while the brake power applied to the rear wheels is controlled only by the fluid pressure braking force. In this technique, if the rear wheels are liable to be locked ahead of the front wheels during performing the regenerative and cooperative brake controls, the wheel cylinder fluid pressure of the rear wheels (i.e., the fluid pressure brake power of the rear wheels) is held by executing the front/rear brake power proportioning control. During executing the front/rear brake power proportioning control, the fluid pressure brake power of the rear wheels is thereby maintained at the value in the start time of the front/rear brake power proportioning control.
In this apparatus, during execution of the front/rear brake power proportioning control, even when the brake-pedal operation demands a braking force larger than that of the brake-pedal operation at the start time of the front/rear brake power proportioning control (referred to as “additional depression” below), the fluid pressure brake power applied to the rear wheels is maintained at the value in the start time of the front/rear brake power proportioning control.
Thereby, after the additional depression, the fluid pressure brake power applied to the rear wheels is insufficient in comparison with the case where the front/rear brake power proportioning control is not executed (the case where the front/rear brake power proportioning control is executed and the brake-pedal operation demands the same brake power as that after the additional depression). In other words, the total brake power (the sum of the braking forces applied to front wheels and the braking force applied to rear wheels) after the additional depression is insufficient in comparison with the case where the front/rear brake power proportioning control is not executed.
Accordingly, in this case, characteristics of the total braking force relative to the brake-pedal depressing force is insufficient for predetermined target characteristics, so that the optimum braking force relative to the brake-pedal depressing force cannot be maintained. It is therefore preferable that when the additional depression is performed during execution of the front/rear brake power proportioning control, the shortage of the fluid pressure brake power applied to the rear wheels (i.e., the total braking force) be compensated.